Fluidized bed combustion system and process for operating same

Abstract: A fluidized bed combustion system and process is disclosed, in which full load stoichiometry is maintained at lower loads. The system and process includes an enclosure (10) and a partition (20) disposed in the enclosure to define a furnace section (36) and an overfire air plenum (38) section. A bed of combustible particulate material is formed in the furnace section (36) and air is passed into the bed in quantities sufficient to fluidize the material and insufficient to completely combust the material. Overfire air is passed into the overfire air plenum, through control dampers (34) in the partition and into the furnace section in quantities sufficient to completely combust the material.

Description: This invention relates to a fluidized bed combustion system and a process of operating same and, more particularly, to such a system and process in which dampers control upper furnace solids loading while maintaining full load stoichiometry at lower loads. Fluidized bed combustion systems are well known and include a furnace section in which air is passed through a bed of particulate material, including a fossil fuel, such as coal, and an adsorbent for the oxides of sulphur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relationship to the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulphur adsorption and low nitrogen oxide emissions. The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of systems utilize a "circulating" fluidized bed in which the fluidized bed density is below that of a typical bubbling fluidized bed, the fluidizing air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith. Circulating fluidized beds are characterized by relatively high internal and external solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, stabilizing the sulfur emissions at a low level. The high external solids recycling is achieved by disposing a cyclone separator at the furnace section outlet to receive the flue gases and the solids entrained thereby from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace through a seal pot or seal valve. All of the fuel is combusted and the heat of combustion is absorbed by water/steam-cooled tube surfaces forming the interior boundary of the furnace section and the heat recovery area. The recycling improves the efficiency of the separator, and the resulting increase in the efficient use of sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. In these type of arrangements, the amount of primary air supplied to the fluidized bed must be limited to that below the ideal amount for complete combustion in order to reduce nitrous oxide (NO x ) emissions. Thus, overfire or secondary air is injected above the fluidized bed in sufficient quantities to maintain a ratio of primary air to secondary air to insure complete combustion. However, problems arise in maintaining this requisite ratio of primary air to secondary air during low load conditions. More particularly, as load is reduced the solids circulation, or loading is also reduced which reduces the residence time of the solids and the capture of sulphur oxides (SO 2 ). Our solution to this is to increase the amount of primary air. However, this destroys the requisite ratio of primary air to secondary air, resulting in increased NO x emissions. Also in these types of fluidized beds, particulate fuel of a size extending over a relative wide range is utilized. For example, a typical bed will contain relatively coarse particles of 350-850 microns in diameter which tend to form a dense bed in the lower furnace, and relatively fine particles of 75-225 microns in diameter which are entrained by the flue gases and recycled. This tends to reduce coarse particle entrainment and cause instability in the dense bed of coarse materials resulting in sluging or choking of the bed material and pressure fluctuations in the lower furnace. According to the invention in one aspect there is provided a fluidized bed combustion system comprising an enclosure a partition disposed in the enclosure to define a furnace section and an overfire air plenum section, the partition having a plurality of openings formed therein for receiving overfire air and discharging it into the overfire air plenum section, a plurality of ducts, through the partition, a bed of combustible particulate material formed in the furnace section and means for passing fluidizing air into the bed in quantities sufficient to fluidize the material and insufficient to completely combust the material, characterised in that a damper is associated with at least one duct for controlling the flow of the overfire air into the furnace section to complete the combustion of the material. According to the invention in another aspect there is provided a fluidized bed combustion method comprising supporting a bed of combustible material in a furnace section, passing fluidizing air into the bed in quantities sufficient to fluidize air into the bed in quantities sufficient to fluidize the material and insufficient to completely combust the material, passing overfire air into an overfire air plenum and then into the furnace section, controlling the passage of the overfire air into the furnace section to establish a predetermined ratio of primary air to overfire air so that the material is completely combusted, characterised in that, when the load on the furnace section is reduced, the amount of air passed into the bed is reduced while the passage of the overfire air into the furnace section is increased to maintain the complete combustion of the material. In such a system and process the requisite ratio of primary air to secondary air can be maintained during low load conditions, thus ensuring that NOX emissions and SO 2 capture are the same as during full load conditions. Also the solids circulation, or loading need not be reduced to unacceptable levels. The introduction of secondary or overfire air can be precisely controlled to maintain the requisite ratio of primary air to secondary air to reduce NO x emissions while allowing for sufficient solids circulation and entrainment to ensure adequate capture of SO 2 . Also stoichiometry can be maintained at lower loads without a significant reduction in solids loading. Further, lower furnace sluging or choking can be reduced. According to the various embodiments of the invention a second partition is disposed within the overfire air plenum to define a recycle section. A separating section receives a mixture of flue gases and entrained particulate material from the furnace section and separates the entrained particulate material from the flue gases. Means pass the separated material from the separating section to the recycle section and from the recycle section back to the furnace section. Means fluidize the recycle section. Means introduce air across the furnace section at varying velocities to induce the flow of the separated material from the recycle section to the furnace section. Openings formed in the partitions permit the separated solids to pass from the recycle section to the furnace section. At least a portion of the walls of the enclosure are formed by tubes, and fluid flow circuit means pass fluid through the tubes to transfer heat generated in the furnace section to the fluid. The flow circuit means further comprises means for passing the fluid in a heat exchange relation to the separated material in the recycle section to transfer heat from the separated material to the fluid to control the temperature of the separated material passed from the heat exchange compartment to the furnace section. The invention will now be more fully described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation depicting the system of the present invention; and Fig. 2 is an enlarged cross-sectional view taken along the line 2-2 of Fig. 1. The drawings depict the fluidized bed combustion system of the present invention used for the generation of steam and including an upright water-cooled enclosure, referred to in general by the reference numeral 10, having a front wall 12, a rear wall 14, and two sidewalls. For clarity only walls 12 and 14 are shown. The walls of the enclosure 10 are formed by a plurality of tubes interconnected by elongated fins to form a contiguous, air-tight structure in a conventional manner. The upper portion of the enclosure 10 is closed by a roof 16 and the lower portion includes a floor 18. A partition 20 is disposed in the enclosure 10 and extends between the front wall 12 and the rear wall 14. The partition 20 is formed by a plurality of finned tubes bent inwardly from the rear wall 14 and plates (not shown) are inserted between the bent tubes to form an air-tight connection along their lengths. The partition 20 includes a vertical portion 20a extending from the floor 18 and parallel to the wall 12, and an angled portion 20b extending from the upper end of the vertical portion to the rear wall 14. A second partition 22 is disposed beneath the partition 20 and is also formed by bending a plurality of tubes out of the vertical plane of the rear wall 14. The partition 22 consists of three portions. The first portion 22a extends up from the floor 18 parallel to, and spaced from, the vertical partition 20a. The second portion 22b extends from the top of the vertical portion 22a and is angled towards and abuts the intersection of the partitions 20a and 20b. The third portion 22c extends between the upper portion of 22b and the rear wall 14. As shown in Figs. 1 and 2, an array of three levels of openings 30a, 30b, and 30c are provided in the angled partition 20b. The center openings 30b are at a staggered pitch with respect to the openings 30a and 30c. A series of ducts 32a, 32b, and 32c register with the openings 30a, 30b, and 30c, respectively. A plurality of dampers 34a are disposed within the ducts 32a and a plurality of dampers 34b are disposed within the ducts 32c. The dampers 34a and 34b are mechanically linked by a common damper control mechanism 35 in a conventional manner and the operation of the dampers 34a and 34b, and the control mechanism 35 will be discussed later. The enclosure 10 is divided into a furnace section 36, an overfire or secondary air plenum 38, and a recycle section 40 by the partitions 20 and 22, with the walls 20a, 22a, and 22b defining an overflow section 42 which will be described in detail. A coal feeder system 44 is provided adjacent to and extends through the rear wall 14. The coal feeder system 44 is supported by the angled partition 22c and registers with openings 20c in the angled partition 20b for introducing particulate material containing fuel into the furnace 36. Because the feeder system 44 operates in a conventional manner to spread the fuel into the lower portion of the furnace section 36, it will not be described in any further detail. It is understood that a particulate adsorbent material can also be introduced into the furnace section 36 for absorbing the sulphur generated as a result of the combustion of the fuel. This adsorbent material may be introduced through the feeder system 44 or independently through openings in any of the enclosure walls (not shown). A water cooled plate 50 extends across the lower portion of the enclosure 10. A plurality of vertically extending air distributor nozzles 52 are mounted in corresponding openings formed in the plate 50. The plate 50 is spaced from the floor 18 and together with walls 12, 20a, 22a, and 14 define air plenums 56a - 56c, respectively. The floor 18 and the plate 50 extend beyond the rear wall 14 to form an air plenum 56d. A horizontal plate 14c extends from the rear wall 14 in a spaced relationship to the plate 50 to define an inlet conduit 57. The air plenums 56a - 56d are adapted to receive air from external sources (not shown) via conduits 58a - 58d, respectively, and selectively distribute the air through the nozzles 52 as needed. The particulate fuel and adsorbent material (hereinafter termed "solids") in the furnace section 36 is fluidized by the air from the plenum 56a as the air passes upwardly through the plate 50. Each nozzle 52 is of a conventional design and, as such, includes a control device to enable the velocity of the air passing therethrough to be controlled. This air promotes the combustion of the fuel in the solids and the resulting mixture of combustion gases and the air (hereinafter termed "flue gases") rises in the furnace section 36 by forced convection and entrains a portion of the solids to form a column of decreasing solids density in the furnace section to a given elevation, above which the density remains substantially constant. Air is selectively introduced into the overflow section 42, the recycle section 40, and the inlet conduit 57, via the coresponding, nozzles 52 as described in our U.S. Patent No. 5,054,436. A conventional cyclone separator 60 extends adjacent to the enclosure 10 and is connected thereto via a duct 62. The duct 62 extends from an outlet opening 14a provided in the rear wall 14 of the enclosure 10 to an inlet 62a provided through the separator wall. A hopper portion 60a extends downwardly from the separator 60. The separator 60 receives the flue gases and the entrained solids from the furnace section 36 in a manner to be described and operates in a conventional manner to disengage the solids from the flue gases due to the centrifugal forces created in the separator. The separated solids in the separator 60 pass downwardly, by gravity, into the hopper portion 60a from which they pass, into and through a dipleg 64 and into the inlet conduit 57. The separated solids then pass from the inlet conduit 57 into the recycle section 40, through an opening 14b provided in the lower portion of the rear wall 14. The solids then pass to the overflow chamber 42 via an opening 22d provided in the partition 22b and then to the furnace 36 through an opening 20d provided in the partition 20a. The actual process of recycling the material is described in the above-identified patent. A pair of vertically spaced overfire air ducts 66 and 68 (Fig. 1) register with openings in the rear wall 14 for introducing overfire or secondary air into the air plenum section 38 and the recycle section 40, respectively. Although not clear from the drawing, it is understood that the tubes forming the partition 22c have no fins so that overfire or secondary air from the duct 68 can pass into the plenum 38. It is understood that a steam drum (not shown) is located above the enclosure 10 and a plurality of headers (not shown) are disposed at the ends of the various walls and partitions described above. Also, a plurality of downcomers, pipes, risers, headers etc. are utilized to establish a steam and water flow circuit. In operation, the solids are introduced into the furnace section 36 through the feeder system 44, via the openings 20c. Alternately, adsorbents may also be introduced independently through openings (not shown) in the enclosure walls. Air from an external source is introduced into the plenum 56a extending below the furnace section 36. The air passes through the nozzles 52 disposed in the furnace section 36 at a sufficient quantity and velocity to fluidize the solids in the latter section and form a circulating fluidized bed as described above. Each nozzle 52 is adjusted so that the velocity of the air discharged therefrom increases from right-to-left as viewed in Fig. 1, i.e., the nozzles closest to the wall 12 discharge air at a relatively high velocity while the nozzles closest to the vertical partition 20a discharge air at a relatively low velocity. A lightoff burner (not shown), or the like, is provided to ignite the fuel material in the solids, and thereafter the fuel material is self-combusted by the heat in the furnace section 36. Thus, the flue gases pass upwardly through the furnace section 36 and entrain a majority of the solids. The quantity of the air introduced, via the air plenum 56a, through the nozzles 52 and into the interior of the furnace section 36 is established in accordance with the size of the solids so that a circulating fluidized bed is formed, i.e., the solids are fluidized to an extent that substantial entrainment is achieved. This occurs in the upper portion of the furnace section 36 and in that area of the lower portion of furnace section closer to the front wall 12, while a relatively dense bed of course material is formed in the lower portion of the furnace section 36. The flue gases passing from the latter area into the upper portion of the furnace section 36, as shown by the flow arrow A, are substantially saturated with solids. However, in that area of the furnace section 36 closer to the partition 20a, some of the relatively course solids disengage from the flue gases due to the relatively low discharge velocities of the nozzles 52 in the latter area as shown by the flow arrows B and C. A large portion of the disengaged solids fall on the angled partition wall section 20b with the greatest concentration near the lower portion of the partition and slide back into the dense bed in the lower portion of the furnace section 36 where they mix with the solids returning to the furnace section 36 from the recycle section 40 as described. The quantity of air introduced into the furnace section 36 through the nozzles 52 in the above manner is controlled so that it is less than that required for complete combustion of the fuel particles. Overfire or secondary air is supplied by the ducts 66 and 68 to the plenum 38 from which the air passes into the furnace section 36 via the ducts 32a, 32b, and 32c, under the control of the dampers 34a and 34b. Thus, overfire air is supplied in sufficient controlled quantities to complete combustion and maintain optimum stoichiometry and upper furnace loading. The upper furnace loading is controlled by controlling the position of the upper and lower dampers, 34a and 34b. As the furnace load is reduced the positions of the upper and lower dampers are adjusted to maintain the desired upper furnace loading with respect to furnace load. The saturated flue gases in the upper portion of the furnace section 36 exit into the duct 62 and pass into the cyclone separator 60 where the solids are separated from the flue gases. The separated solids pass from the separator 60 through the dipleg 64 and are recycled, via the section 40 to the furnace section 36. The following advantages are achieved by the process and system of the present invention: 1. Because the overfire air is discharged, via the ducts 32a, 32b, and 32c through the angled partition section 20b, which, in effect, is located near the cent of the enclosure 10, the mixing of the overfire air, the primary air from the nozzle 52 and the fuel particles, is enhanced resulting in increased combustion of the fuel particles. 2. Air is introduced into the plenums 56b-56d in sufficient amounts to maintain a fluidized state to ensure flow from the inlet conduit 57 to the furnace section 36. 3. The dampers 34a and 34b allow the furnace to achieve the same NO x emissions performance at part load conditions as at full load conditions. 4. The angled partition wall section 20b provides a "return slide" for the disengaged course material which enhances mixing and avoids choking of the circulating solids. 5. The introduction of secondary or overfire air from the plenum 38 is precisely controlled by the dampers 34a and 34b to maintain the requisite ratio of primary air to secondary air. Thus, reducing NO x emissions while allowing for sufficient solids circulation and entrainment to ensure adequate capture of SO 2 . 6. Stoichiometry is maintained at lower loads by reducing the amount of primary air introduced into the furnace 36 through the nozzles 52 and increasing the amount of overfire air flowing through the dampers 34a and 34b and the partition 20b.

Claims: A fluidized bed combustion system comprising an enclosure (10) a partition (20) disposed in the enclosure (10) to define a furnace section (36) and an overfire air plenum section (38), the partition (20) having a plurality of openings (66, 68) formed therein for receiving overfire air and discharging it into the overfire air plenum section (38), a plurality of ducts, (32a, 32b, 32c) through the partition (20), a bed of combustible particulate material formed in the furnace section (36) and means (52) for passing fluidizing air into the bed in quantities sufficient to fluidize the material and insufficient to completely combust the material, characterized in that a damper (34a, 34b) is associated with at least one duct (32a, 32b, 32c) for controlling the flow of the overfire air into the furnace section (36) to complete the combustion of the material. A system as claimed in Claim 1 further comprising a separating section (60) for receiving a mixture of flue gases and entrained particulate material from the furnace section (36) and separating the entrained particulate material from the flue gases. A system as claimed in Claim 2 in which a second partition (22) is disposed within the overfire air plenum section (38) to define a recycle section (40). A system as claimed in Claim 3 in which means are provided for passing the separated material from the separating section (60) to the recycle section (40) and from there back to the furnace section (36). A system a claimed in either Claim 3 or Claim 4 in which means (32) are provided for fluidizing the material in the recycle system (40). A system as claimed in Claim 5 in which the means (52) for passing fluidizing air introduce that air across the furnace section (36) at varying velocities to induce a flow of the separated material from the recycle section (40) back to the furnace section. A system as claimed in any preceding claim in which an openings (20d, 22d) formed in the partitions (20,22) permit separated solids to pass from the recycle section (40) to the furnace section (36). A fluidized bed combustion method comprising supporting a bed of combustible material in a furnace section (36), passing fluidizing air into the bed in quantities sufficient to fluidize the material and insufficient to completely combust the material, passing overfire air into an overfire air plenum (38) and then into the furnace section (36), controlling the passage of the overfire air into the furnace section (36) to establish a predetermined ratio of primary air to overfire air so that the material is completely combusted, characterised in that, when the load on the furnace section (36) is reduced, the amount of air passed into the bed is reduced while the passage of the overfire air into the furnace section (36) is increased to maintain the complete combustion of the material. The process of Claim 8 further comprising discharging a mixture of flue gases and entrained material from the furnace section, separating the entrained material from the flue gases, passing the separated flue gases to a heat recovery section, passing the separated material into and through a recycle section, and varying the velocities of the fluidizing air along different locations so that the separated material is drawn from the recycle section back into the furnace section. The process of Claim 9 in which the separated material passes the recycle section into an area of the furnace section adjacent to the recycle section, and in which the step of varying comprises the step of fluidizing the material in that said area of the furnace section at a lower velocity than the velocity of the air in the remaining portion of the furnace section to cause the separated material to flow from the recycle section to the furnace section. A process as claimed in Claim 10 in which the velocity of the fluidizing air introduced to the furnace section progressively increases in a direction from the said area across the furnace section to cause the separated material to flow from the recycle section to the area of the furnace section.

Inventors: CAMPBELL WALTER ROBERT, DIETZ DAVID HAROLD, CAMPBELL, WALTER ROBERT, DIETZ, DAVID HAROLD
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1994
Main classification: F23C11
IPC codes: F23C10, F22B31